The use of lightguide communications involving the use of optical fibers has grown at an unprecedented pace. Typically, an optical fiber has a diameter on the order of 125 microns, for example, and is covered with a coating material which increases the outer diameter of the coated fiber to about 250 microns, for example. Optical fiber cables may comprise a plurality of these optical fibers which are stranded together or which are assembled in planar arrays which are referred to as ribbons.
The technology for forming low-loss optical fibers, which is shown for example in U.S. Pat. No. 4,217,027 which issued on Aug. 12, 1980 in the names of J. B. MacChesney and P. B. O'Connor, has advanced to a point where there is widespread commercial manufacture of optical fibers. Most processing includes drawing an optical fiber from a previously manufactured glass boule, sometimes referred to as a preform. During the drawing process, the fiber is usually coated with a protective, curable material which may be cooled or cured thermally, by radiation, or by other suitable techniques for achieving solidification.
The introduction of optical fiber applications to evermore hostile environments, such as in underwater cable, has required that more stringent requirements be imposed on physical properties of the fiber, such as strength. For these more demanding applications, as well as for other less demanding ones, it has become increasingly more common to splice optical fibers which have broken, either accidentally, or during appropriate proof testing. Additionally, extremely long lengths of fiber may be obtained by splicing a plurality of lengths which are obtained using current manufacturing techniques. For these and other applications, splicing in which the coating material is removed from end portions of two fibers which are then fused together end to end provides a viable means for joining the ends of two glass fibers with an acceptable loss. However, the recoating of the bared spliced fiber end portions continues as a problem to be overcome, especially while maintaining stringent requirements on dimensional and strength parameters associated with the coated fiber.
A method of recoating spliced end portions of optical fibers is disclosed in U.S. Pat. No. 4,410,561 which issued on Oct. 18, 1983 in the name of A. C. Hart, Jr. The method involves placing the spliced fiber end portions and adjacent portions within a cavity in the form of a groove such as a semicircular or V-groove in a split mold. The effective diameter of the groove is somewhat greater than that of the remaining coated portion of each fiber. The fibers are positioned so that only the coated portions of the fibers touch the surface which defines the groove, while the vulnerable, uncoated spliced end portions of the fibers remain suspended and do not contact the groove surface. Then, the mold is covered to enclose the groove and a suitable curable coating material is injected into the groove to recoat the bared, spliced fiber end portions and contact the adjacent originally coated portions of the spliced fibers. The coating material is then cured to yield a recoated splice section with a transverse cross-section which is essentially identical to that of the original coated fiber.
This patented molding process provides a recoated splice; however, it has been determined that bubbles may occur in the recoating. The existence of bubbles may lead to stress concentrations when the fiber is handled subsequently. This is particularly undesirable in underwater cables where splices are inaccessible and under stress for many years. Long term integrity of the fiber may also be affected by the failure of the recoating material to overlap the original coating material on the portions of the fibers adjacent to the spliced end portions.
It has been determined that there are three sources of bubbles. These are air already present in the recoating material, air entrained during the molding process, and bubbles formed during the shrinkage of the recoating material during its cure. The bubbles due to shrinkage tend to be concentrated at the interface between the coating on the unbared fiber portions and the recoating material. This is caused by the pulling away of the recoating material from the coating material on the unbared fiber portions during curing. Because of the relatively long length of the portions to be recoated compared to the cross-section of the coated fiber and because the mold cavity is enclosed completely, the longitudinal shrinkage cannot be compensated for by uninhibited contraction transverse to the longitudinal axes of the fiber end portions. The coating material contacts and tends to adhere to an immovable surface around the entire periphery of the mold cavity which inhibits its contraction. Also, surface craters can derive from bubbles that break apart or in the recoating material that pulls away from the mold irregularly because of surface adhesion during cure. Craters over 1-2 mils in depth approach the surface of the fiber and may not protect it adequately from damage.
Inadequate overlap between the recoating material and the original coating material on the unbared portions of the fibers is another problem. It may result in the separation of the existing and recoating materials and expose the bare fiber.
As should be evident, there is still a need for methods and apparatus which may be used to recoat bared end portions of optical fibers which comprise a splice and to do so in a manner which avoids the formation of bubbles. Further, the methods and apparatus must be effective to provide continuity of coating material at the ends of the splice where the original coating material on the unbared portions of the coated fibers meets the recoating material.